Ah, well, that's interesting...
So most themes usually access the excerpt using
get_the_excerpt() or something similar (which constructs and excerpt if one hasn't been entered), but for some reason, your theme author decided to use
get_the_content() and trim the full content himself instead! Since you add social buttons to the top of your content, your theme strips the html, but the twitter button also contains some text (the word "Tweet"), so that gets left behind.
It's always better for authors to use standard WordPress functions and methods, or you get weird behavior like this.
Luckily the "Tweet" text is rather optional and unnecessary, so I've removed it. There's no point in creating a new version of the plugin for this, so if you uninstall and reinstall the plugin, you should be good. Don't forget to check the Preserve on Uninstall option before removing the plugin. ;-) (see http://surniaulula.com/codex/plugins/nextgen-facebook/how-to/uninstall-the-plugin/).